1. Field of the Invention
The invention relates to an extraction agent composition for the selective extraction of one or more metals from a metal-containing aqueous solution in an organic medium and particularly to an extraction agent for the selective extraction of nickel (II) and cobalt from an aqueous solution.
2. Description of the Prior Art
Great attention has constantly been paid to the separation of the valuable metals nickel and cobalt in metallurgy. This separation has, however, proven very difficult to control. Among the alternative hydrometallurgical separation processes which have had some success are, primarily, the methods according to which the separation of the metals in question takes place in a strong chloride environment. Depending on the actual ores and their concentrates, the recovery of metals is mainly based on sulphates, i.e. the metals which are present are separated at the extraction stage in the form of their sulphate salts.
In addition to nickel and cobalt, the extraction solution in question often contains iron, copper, and zinc. At the solution purification stage it is not difficult to remove the latter metals from sulphate solutions. With the exception of the most modern extraction processes, copper and zinc, for example, are removed in the form of sulphides by precipitation, in which case the ferric iron which is present is simultaneously reduced into a bivalent state, which is often advantageous considering the further treatment. The problem now to separate the nickel (II) and cobalt (II) which are present has been solved according to various patents, such as U.S. Pat. Nos. 3,085,054, 3,128,156, 3,131,998, 3,194,652 and Duth patent No. 6,600,609, by using a strong chloride environment, i.e. in most cases it has been necessary to carry out an anion exchange from a sulphate into a chloride. It has been suggested that this can be achieved with a cation exchange type extraction agent such as for example, light petroleum. After the nickel and cobalt have been extracted from the sulphate solution in question into said extraction agent, the metals are removed from the latter by means of a strong hydrochloric acid (6-9 M), at which time the di-nonyl-naphthalene-sulphonic acid becomes simultaneously regenerated. From such a hydrochloric acid solution the cobalt can be extracted selectively by means of various amines or ammonium salts, preferably tertiary amines or quaternary ammonium salts. Such an anion exchange type extraction reagent binds the formed tetrachloride cobaltate, while nickel, which does not form an analogous complex, does not end up in the organic phase. The extracted cobalt can be simply removed from the organic phase by bringing the phase in contact with water, which rapidly causes a hydrolytic decomposition of the chloride cobaltate which is present. Thereafter both the nickel and the cobalt can be recovered from the solutions by known methods. An interesting possibility of combining the recovery of nickel and cobalt with their separation from each other is offered by the autoclave method, which is also based on chlorides. Promising results have been reported in regard to the selective reduction of nickel with gaseous hydrogen in an autoclave, followed by the reduction of cobalt under somewhat altered reduction conditions.
The separation of nickel (II) and cobalt (II) can thus be achieved in an acid chloride environment in which the chloride content has been raised to value 200-300 g Cl.sup.- /l in most cases. Because such chloride solutions are highly corrosive, the use of a chloride requires a treatment apparatus which is as corrosion-resistant as possible. Usually the purpose is not to perform the entire metal recovery process on a chloride basis but the idea has been to proceed to an appropriate chloride environment only at the nickel/cobalt separation stage. Because then, due to corrosion as well as purely practical factors, the use of both sulphate and chloride solutions in the same solution purification process requires special measures, it is very important to be able to carry out the difficult separation of nickel and cobalt from each other without the presence of a chloride, preferably regardless of the type of the counter-ions.
Even though in most cases it has been possible to achieve the desired separation through extraction, unexpectedly great problems have been faced in finding a suitable extraction agent for separating nickel (II) and cobalt (II) from, for example, sulphate-based water solutions. This has, however, succeeded with the use of complicated, often unstable, extraction reagents with a questionable technological value. However, the results which have been achieved with acid thiophosphates according to U.S. Pat. No. 3,336,133 have proven promising. In another suggested separation process, U.S. Pat. No. 3,276,683, the chloride has been eliminated by oxidizing the cobalt with oxygen, air, hydrogen peroxide, ammonium peroxide, or some other similar reagent into a trivalent state, preferably in an ammoniacal environment and by then performing a relatively selective extraction of nickel (II) with .alpha.-hydroxy-oxime for the extraction agent. Thereafter the extracted nickel is separated from the organic phase by means of a strong acid solution or an ammonia solution. However, some serious disadvantages seem to be linked to this separation process, some of them being the slow oxidation of cobalt with air, equally slow extraction of nickel, low extraction capacity, and poor capability of being regenerated with sulphuric acid solution.
One of the principal objectives of the present invention is to provide an improved extraction agent for the separation of nickel (II) and cobalt (II) from each other from aqueous solutions containing both nickel and cobalt. It has been considered very important that the separation be possible regardless of the type of the counter-ion, i.e., that the said separation must be possible from both sulphate, nitrate, and chloride solutions. Basing the separation on a high chloride content, or a necessity of oxidizing the cobalt which is present into a trivalent state, has been specifically avoided.
The object of the present invention is thus to provide an extraction agent composition for separating metals from an aqueous solutions, an extraction agent composition of which the extractive properties are relatively little dependent on the anion environment in the water solution. According to this invention it is possible to obtain a decisively improved extraction solution for various metal separation processes by adding a small amount of, for example, a strong organic acid to an extraction solution which contains one or more extraction agents. Especially a number of various sulphonic acids have proven highly practicable for this purpose.
A synergetic extraction solution which contains carboxylic acid, hydroxy-oxime, and a small amount of the crucial sulphonic acid is especially practical for the separation of nickel (II) and cobalt (II). It has been noted (D. S. Flett and D. W. West, ISEC 71) that the separation process of nickel (II) and cobalt (II) can hardly be based on an extraction agent composition containing .alpha.-hydroxy-oxime ("LIX-63") and naphthenic acid, in spite of the obtained advantageous nickel/cobalt separation coefficients, because said extraction agent composition for some unknown reason reacts very slowly with nickel. It has been proven, however, that by using the extraction agent composition according to the present invention it is possible to carry out a rapid extraction of nickel with a composition of acids and oximes. If a small amount of a sulphonic acid, such as a sulphonated, alkyl-substituted naphthalene product di-nonyl-naphthalene-sulphonic acid, which is abbreviated DNNS, or "ECA 6414" or a sulphonic acid which contains aliphatic and/or aromatic groups and has a branching molecular structure, is added to an extraction solution which consists of hydroxy-oxime and carboxylic acid in a non-polar solvent, the result is an excellent extraction agent composition for nickel extraction. With this extraction solution composed of three different components, nickel (II) and cobalt (II) can be separated in a process in which the separation of the said metals takes place advantageously according to the counter-current principle by using the separation scheme illustrated in the accompanying FIGURE. In spite of the possible strict purity requirements, the necessary number of stages usually remains relatively low because an extraction agent composition according to this invention makes it possible to achieve such high separation coefficients as 150. The great advantage of this extraction agent composition is that it does not require any definite anion environment but functions excellently in, for example, sulphate-based solutions.